Chemical-mechanical polishing and chemical-mechanical planarization processes, both of which are referred to herein as “CMP” processes, are abrasive techniques that typically include the use of a combination of chemical and mechanical agents to planarize, or otherwise remove material from a surface of a micro-device workpiece (e.g., wafers or other substrate) in the fabrication of micro-electronic devices and other products. A planarizing or polishing pad (“CMP pad”) is a primary component of a CMP system. The CMP pad is used with a chemical solution along with abrasives, which may be present in the solution as a slurry or fixed within the pad itself, to mechanically remove material from the workpiece surface.
FIG. 1 illustrates a conventional chemical-mechanical planarization (CMP) apparatus 10 with a circular table or platen 12, a carrier assembly 14, and a CMP pad 16. The planarizing apparatus 10 can have an under-pad or subpad 17 attached to a surface of the platen 12 for supporting the CMP pad 16. A drive-assembly 18 rotates the platen 12 (indicated by arrow “A”) and/or reciprocates the platen 12 back and forth (indicated by arrow “B”), and the motion provides continuous movement of the CMP pad 16 relative to a workpiece 20 (e.g., a wafer) secured onto a substrate holder or carrier 22. In the illustrated embodiment, an actuator assembly 24 is coupled to the carrier 22 to provide axial and/or rotational motion to the carrier 22 as indicated, respectively, by arrows “C” and “D”. Also as shown, the carrier 22 is coupled by an arm 28 to an actuator 26 that rotates (indicated by arrow “E”) to “sweep” the carrier 22 along a path across the planarizing surface 30 of the CMP pad 16. Although not shown, the carrier 22 can also be a weighted, free-floating disk that slides over the CMP pad 16. Several nozzles 32 attached to the carrier 22 dispense a planarizing solution 34 onto the surface 30 of the CMP pad 16. In FIG. 2, another embodiment of a typical CMP processing apparatus is shown with a delivery system 36 having an arm 38 with a nozzle 32 for delivery of a slurry or planarizing solution 34 over the surface 30 of the CMP pad 16 during a planarizing operation, and nozzles 39 for a high pressure DI rinse to clean the pad.
In operation, the workpiece 20 and/or the CMP pad 16 are moved relative to one another allowing abrasive particles in the pad or slurry to mechanically remove material from the surface of the workpiece 20, and reactive chemicals of the planarizing solution 34 on the surface 30 of the CMP pad 16 to chemically remove the material. This action results in wear of the planarizing surface 30 of the CMP pad 16.
Conventional CMP pads are round or disk-shaped, planar, and have larger dimensions than the workpiece substrate. CMP pads are typically fabricated by forming the pad material into large cakes that are subsequently skived, or sliced, to a desired thickness, or by individually molding the pad. Pads can also be produced as individually molded or with abrasives embedded in the pad (fixed abrasive). The condition of the planarizing surface of the CMP pad is one variable affecting the polishing rate and uniformity of the polished surface if the workpiece substrate.
As shown in FIGS. 3-4, CMP pads have a thickness “T” and a planarizing surface 30. CMP processes use pads that typically include openings 40 (e.g., grooves, channels, perforations) having different grooving styles to improve process performance, which in the illustrated embodiment are in the form of grooves. The depth “D”, width “W”, and pitch “P” (i.e., distance between sidewalls or lands 41 of the grooves, etc.) of the openings 40 (e.g., grooves) affect the wafer to pad contact area and slurry transport across the wafer-surface. This, in turn, affects the uniformity of the planarized surface of the workpiece, the planarizing or polishing rate and capabilities, and defects in the workpiece surface.
Most CMP pads are initially received from the manufacturer with a hydrophobic, non-planar surface. Before use, the planarizing surface of the CMP pad typically undergoes a conditioning process to planarize and abrade (roughen) the surface so that effective planarization of the workpiece surface can be achieved. Typically, a hard CMP pad is conditioned using a rough or abrasive pad, such as a diamond or diamond-on-metal conditioning stone or pad. In some operations, the planarizing pad is removed from the platen and placed on a separate conditioning machine.
The planarizing apparatus 10 illustrated in FIG. 1 includes a second carrier assembly 42 that includes an actuator assembly 44 coupled to a carrier 46 and to an arm 48, which can be actuated to move the carrier 46 axially (indicated by arrow “F”) and/or to rotate the carrier 46 (indicated by arrow “G”) to engage the conditioning surface 50 of a conditioner or conditioning medium 52 against the planarizing surface 30 of the CMP pad 16. Exemplary conditioners 52 include, but are not limited to, pads, diamond brushes, and nylon brushes. The carrier assembly 42 can include an actuator 54 that operates to rotate the arm 48 (indicated by arrow “H”) to move or sweep the conditioning pad 52 in an arcuate sweep path against the planarizing surface 30 of the CMP pad 16 between processing cycles. The conditioning pad 52 abrades the surface 30 of the CMP pad to planarize it, which prevents glazing of the pad surface and provides a fresh surface for polishing.
The condition of the planarizing surface of the CMP pad also changes over time from the collection of residual matter on the planarizing surface of the pad during the CMP operation, which can glaze over sections of the pad surface. The workpieces can also wear depressions into the surface of the CMP pad, resulting in a non-planar processing surface. Typically, a pad is also conditioned after processing of a number of workpieces to remove slurry residue and eliminate surface irregularities (e.g., protrusions, depressions), and restore the surface texture of the pad to a desired condition for planarizing additional workpieces.
Referring to FIGS. 4-5, wear and conditioning of the CMP pad leads to a decrease in the initial depth D1 of the openings (e.g., grooves) to a reduced depth D2, which can affect slurry flow dynamics and affects the useful wear life of the pad. For example, during processing, the flow of solution (slurry) across the pad results in abrasive particles of the slurry settling within the grooves of the processing surface of the pad. Over multiple applications, comparatively fewer particles settle into the grooves as the grooves become shallower, which is accompanied by an increasing amount of abrasive particles being present on the surface of the pad. This effect alters slurry efficiency and polish dynamics with respect to later-processed workpieces. Thus, a gradual reduction of the groove depth of a pad can affect the rate and uniformity of the polishing process over time, which can adversely impact later planarized workpieces. In practice, the number of device polishings for any given pad is tracked and the pad is then replaced after an experimentally determined number of cycles, generally before the pad is completely worn out or would damage the substrate being polished. This is also true in the case of pads with embedded abrasives in the form of posts standing up. The posts with abrasives tend to wear during polishing and conditioning.
One factor determining the life span of a CMP pad (i.e., the number of wafers processed per pad) is the depth “D” of the openings (e.g., grooves) in the CMP pad. For example, if a particular CMP process requires polishing using a pad having a shallow groove structure with the depth D1 of the grooves at about 250 μm (about 10 mil) with a pad wear of about 0.25 μm/wafer (about 0.01 mil/wafer), with a continuing reduction in the groove depth to D2 over time (and an associated reduction in pad thickness to T2), the life of the pad will be only about 600-800 wafers processed. With a pad having a standard thickness T1, of about 50-80 mil (about 1.3-2 mm), the pad should be capable of processing about 6000-8000 wafers with deeper initial grooving. Thus, although required by process specifications, the use of shallow grooves results in underuse of the pad and loss of valuable pad life, as well as the loss of operator time due to the need to repeatedly shut down the CMP apparatus to continually replace the CMP pad.
In manufacturing a pad, the initial depth D1 of the openings (e.g., grooves) are machined to a specified depth into the pad thickness T1, depending on the process requirements. However, there are also certain constraints on how deep the grooves can be formed into the body of the pad.
For example, there is some amount of movement of the lands 41 of the grooves 40 on the pad during a processing operation from the pressure applied by the contact and downforce of the workpiece onto the surface of the pad. Typically, the initial depth (D1) of the openings is shallow extending into only about 30-40% of the total pad thickness (T1) in order to provide a rigid and immovable pad surface for providing an acceptable planarizing effect. Consequently, about 60-70% of the pad thickness is unused.
However, forming the openings (e.g., grooves) deeper into the pad will result in shearing of the lands of the openings when the pad is put into contact with a wafer, rather than the land maintaining a relatively stiff, vertical stance due to a lack of supporting material adjacent to the lands. In addition, deeper grooves without any support can also cause sidewall collapse due to lack of stability and the viscoelastic nature of the pad materials. This limits the initial depth D1 of the openings (e.g., grooves) within the pad.
In addition, deep grooves present pad cleaning challenges. The slurry particles and polishing debris tend to collect in the grooves. As shown in FIG. 2, high pressure DI rinse from the arm 38 using nozzles 32 is delivered to the pad between wafers to clean polishing debris from the pad. If the pad grooves are significantly deep, it poses challenges to pad cleaning. The debris that collects in the grooves can lead to defects on the polishing substrate (wafer) if not cleaned properly. Without properly controlled pad groove depth, it is difficult to clean the pads.
Therefore, it would be desirable to provide a CMP pad and process of planarizing a workpiece that overcomes such problems.